The invention relates to a charged particle device. In particular, this invention relates to a device and method for the examination of specimen with a beam of charged particles.
Beams of negatively or positively charged particles can be used for the examination of specimen. Compared to optical light, the resolving power of a beam of charged particles is several magnitudes higher and allows for the examination of much finer details. Accordingly, charged particle beams, especially electron beams, are used in a variety of ways in biology, medicine, the materials sciences, and lithography. Examples include the diagnosis of human, animal, and plant diseases, visualisation of sub cellular components and structures such as DNA, determination of the structure of composite materials, thin films, and ceramics, or the inspection of masks and wafers used in semiconductor technology.
The two basic types of charged particle devices for the examination of specimen, that are in widespread use today, are the transmission electron microscope (TEM) and the scanning electron microscope (SEM). In addition to the normal use the two microscopes, both the TEM and SEM have been modified, resulting in instruments designed to perform specific functions. For example, the scanning transmission electron microscope (STEM) produces a transmitted image, as a TEM does, but uses a scanning beam, as the SEM does.
In conventional charged particle devices like, for example, a scanning electron microscopes (SEM) the designer always had to make a compromise between the arrangement of the objective lens for focusing the particle beam onto the specimen and the arrangement of the detector, because it is preferable to arrange both the objective lens and the detector as close as possible to the specimen in order to get the best results. However, due the fact that detectors can not be minimised beyond a certain degree, there is just not enough space for the detector in the vicinity of the specimen without negatively affecting the focusing properties of the objective lens.
Furthermore, despite their widespread use, electron microscopes are large and fairly complicated instruments, which in many universities and industrial settings have often become centralised. Electron microscope technicians have specialised training to carry out the day-to-day operations of the laboratory. However, the maintenance of the instrument and especially the adaptation of the instrument to specific measurement needs, for example by use of different spectrometers and detectors, often leads to costly downtimes of the instruments affecting a large number of users.
These problems have been partially addressed in the prior art, e.g., U.S. Pat. No. 5,422,486 which discloses a particle mirror used inside an electron microscope. However, there is a need for additional improvement.
In view of the foregoing, an object of the present invention is to provide an improved charged particle device that exhibits an improved design and that can be more easily adapted for various measurement needs. Another object of the present invention is to provide a particle mirror for use in a charged particle device. Still another object of the present invention is to provide an improved method for the examination of specimen with a beam of charged particles.
According to one aspect of the present invention, there is provided a charged particle device as specified in independent claim 1. According to a further aspect of the present invention there is provided a particle mirror for use in a charged particle device as specified in independent claim 14. According to a still further aspect of the present invention there is provided a method for the examination of specimen with a beam of charged particles as specified in independent claim 19. According to a still further aspect of the present invention there is provided a charged particle device as specified in independent claim 20. Further advantages, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms.
According to the first aspect of the present invention, there is provided a charged particle device comprising: a particle source for providing a charged particle beam; an objective lens for focusing the particle beam onto a specimen, said objective lens having an optical axis; a particle mirror located on the optical axis of the objective lens, said particle mirror having a front surface, a back surface, a drift region reaching from the back surface to die front surface for letting the charged particle beam pass from the back surface to the front surface, said drift region being positioned away from the optical axis, and a deflecting region located on the front surface for deflecting charged particles coming from the specimen towards a detector.
The improved charged particle device has the advantage that the use of a particle mirror results in additional freedom for the design of the charged particle device. A particle mirror, contrary to a particle detector, can be arranged in the device more easily without negatively affecting the focusing/projecting properties of the objective lens. By having the drift region, the particle mirror can be integrated in the device more easily, without negatively affecting charged particle beam. Due to the fact that the drift region is positioned away from the axis of the objective lens, the area, where the axis intersects the mirror, can be used as deflecting region, which increases the quality of the examination considerably, because the particles moving along the axis carry an important part of the information about the specimen.
Furthermore, since there is now no limitation on the size of the detector, all kinds of detectors and spectrometers can be used to analyse the specimen. In addition to that, one type of detector can be easily replaced by another type of detector, in order to adapt the device to specific measurement needs.
According to a preferred embodiment, the particle mirror comprises a deflecting region located on the front surface for deflecting all particles in a given velocity range (energy range) and in a given angular range, so that the angle xcex2o between the outgoing path of the particle and the axes normal to the front surface of the mirror, at the point where the particle hits the mirror, equals the angle xcex1xcex2i between the incoming path of the particle and the axes normal to the front surface of the mirror. Due to the fact that the energy and the angular distribution of the particles coming from the specimen is preserved by the mirror, the detector is capable of basically recording the same information as if it were directly located near the specimen.
According to a preferred embodiment, the drift region of the particle mirror reaching from the back surface to the front surface is positioned away from the center of the mirror. The particle mirror is preferably arranged in such a manner, that most of the particles coming from the specimen are deflected towards the detector. This implies that the geometric center of the mirror is preferably located where the particles coming from the specimen are concentrated.
According to a preferred embodiment, the charged particle device further comprises a deflection unit for directing the charged particle beam essentially along the optical axis of the objective lens, said deflection unit being arranged between the particle mirror and the objective lens. In this arrangement basically all particles coming from the specimen and moving along the optical axis can be deflected to the detector. Obviously, the deflecting unit affects the charged particle beam moving towards the specimen as well as the charged particles coming from the specimen. However, these two types of particles are affected in different manner, which leads to a separation of the two types of particles. This is regardless of the deflection unit being magnetic or electrostatic.
A magnetic deflection unit, in particular, separates charged particle beam moving towards the specimen and the charged particles coming from the specimen into complementary parts. According to Lorentz Law, charged particles coming from the specimen which fly in a direction opposite to the direction of the charged particle beam experience an opposite force. In other words, they are directed into a region which is complementary to the region of the primary particle beam.
In case the deflection unit is electrostatic, the charged particles coming from the specimen are directed into the region as the primary particle beam. However, the angle of redirection of the charged particles coming from the specimen is bigger since the angle of redirection is inversely proportional to the velocity. This results in a directional separation of the secondary particles from the primary particles. Since a magnetic deflection unit deflects charged particle beam moving towards the specimen and the charged particles coming from the specimen into opposite directions, it is preferred to use a magnetic deflection unit.
According to a still further preferred embodiment, the charged particle device further comprises a three step deflection unit for deflecting the charged particle beam away form the optical axis and back onto the optical axis. By using a three step deflection unit the particle source, and any other element used to shape the beam of charged particles, can be arranged along the axis of the objective lens, which leads to compact design of the complete device. The three step deflection unit may consist of elements, like magnetic coils, which are used only for this purpose. However, the three step deflection unit may also consist of elements, which, for example, are additionally used to move the charged particle beam across the specimen (scanning unit).
According to a still further preferred embodiment, the particle mirror is tilted with regard to the optical axis by an angel xcex1 between about 20 and about 70 degree, preferably about 40 and about 50 degree, most preferably about 45 degree. By this orientation of the particle mirror, the particles coming from the specimen are easily directed to the periphery of the device, where all kinds of detectors can be arranged without affecting the rest of the device. By using a 45 degree orientation of the particle mirror, it can be assured that the time difference of two different particles moving to the detector is minimal, as long as there are of the same initial velocity.
According to a still further preferred embodiment, the particle mirror comprises a conductive surface or a conductive deflecting grid kept on a predetermined potential sufficient to deflect all particles having less than a predetermined energy. Thereby, the surface of a metal plate can be used as a conductive surface. Furthermore, a ceramic material, for example Al2O3, having a conductive coating can be used for this purpose. In this arrangement the particle mirror acts like a low pass filter and by varying the potential of the conductive surface or the conductive deflecting grid, the mirror can be used for spectroscopic purposes. Furthermore, it is preferred if the particle mirror comprises at least one conductive screening grid for screening potential of the surface or the conductive deflecting grid from the rest of the device and/or a particle absorber for absorbing particles having more than the predetermined energy. In case a metal plate or a ceramic plate having a conductive coating is used, the metallic or ceramic plate can be used as the particle absorber.
According to a still further preferred embodiment, a second detector is arranged behind the conductive deflecting grid for detecting particles having more than the predetermined energy.
According to a still further preferred embodiment, the charged particle device further comprises a high pass filter, which is arranged in front of the detector, allowing only particles having an energy above a predetermined energy to enter the detector. The combination of a low pass filter and a high pass filter allows to pick out any band of energies for the detection and thus enhance material contrast or other features of the specimen. Thereby, it is preferred that the high pass filter comprises a conductive filtering grid kept on a predetermined potential sufficient to filter out all particles having less than a predetermined energy.
According to a still further preferred embodiment, said drift region is positioned away from the optical axis of the objective lens, so that all charged particle coming from the specimen within an angle xcex3xe2x89xa65 degree, preferably xe2x89xa610 degree, as measured from the optical axis of the objective lens, hit the deflecting region of the mirror. This has the advantage that, especially for specimen with a high topography, most of the charged particles coming from the specimen can be detected.
According to the second aspect of the present invention, there is provided a particle mirror for use in a charged particle device comprising: a front surface and a back surface, a deflecting region located on the front surface for deflecting all particles in a given velocity range and in a given angular range, a drift region reaching from the back surface to the front surface for letting particles pass from the back surface to the front surface of the mirror, the drift region being positioned away from the geometrical center of the mirror.
According to a preferred embodiment, the particle mirror further comprises a conductive surface or the conductive deflecting grid kept on a predetermined potential sufficient to deflect all particles having less than a predetermined energy. Furthermore, it is preferred if the particle mirror comprises a conductive screening grid for screening potential of the surface or the conductive deflecting grid from the rest of the device and/or a particle absorber for absorbing particles having more than the predetermined energy.
According to the third aspect of the present invention, there is provided a charged particle device comprising: a particle source for providing a charged particle beam; an objective lens for focusing the particle beam onto a specimen; a particle mirror for deflecting charged particles coming from the specimen towards a detector, and a high pass filter being arranged in front of the detector, allowing only particles having an energy above a predetermined energy to enter the detector.